As top officials at a G8 meeting in Quebec today called for the international community to stop the spread of nuclear weapons, the drumbeat for Iran nuclear sanctions is nearing a crescendo.

"We urge a heightened focus and stronger coordinated action, including sanctions if necessary, on the Iranian regime," said Canadian Prime Minister Stephen Harper today. "Tehran must halt its nuclear activities and engage in peaceful dialogue. There is much at stake."

While Iran remains defiant, the United States and its allies are pushing forward – albeit cautiously, in an effort to balance the desire to punish the Tehran government against concerns that sanctions could undermine the country's opposition Green Movement. The US is now calling for the United Nations Security Council to impose targeted sanctions, and is working to wear down the resistance of other Security Council members, especially Russia and China.

The US accuses Iran of attempting to develop nuclear weapons. Iran insists its program is for peaceful purposes.

What sanctions have already been imposed on Iran?

US economic measures against Iran date to the Islamic Revolution in 1979. The sanctions were increased after Iran was implicated in the bombing of the US Marine Corps barracks in Beirut in 1983 and the US added Tehran to the list of state sponsors of terrorism.

US sanctions included freezing Iranian assets, bans on investment in Iran, and prohibiting the export to Iran of certain products. International suspicion about Iran's nuclear program was piqued in 2002 when uranium enrichment and heavy-water production plants were revealed.

Iran's incomplete cooperation with the International Atomic Energy Agency (IAEA) and failure to comply with a Security Council order to stop uranium enrichment led to three rounds of multilateral sanctions from the Council, beginning in 2006. Those resolutions froze assets of individuals and companies associated with Iran's nuclear program, limited travel by certain individuals, and prohibited the export of nuclear- and missile-related technology to Iran.

What effect did those sanctions have on Iran's nuclear program?

Very little. Analysts agree that the Security Council's previous sanctions have not prevented Iran from making progress in its nuclear program.

The IAEA said in 2009 that Iran had accumulated enough low-enriched uranium to make one nuclear bomb if it was much more highly enriched. In September, Iran said it had constructed a second uranium-enrichment plant unknown to international inspectors. President Mahmoud Ahmadinejad announced in February that Iran had completed its first batch of 20 percent enriched uranium, and had the capability to produce weapons-grade uranium, although he said Iran had not done so.

Iran insists the highly enriched uranium is for use in a medical reactor, but experts say that enriching uranium to such a level is an important step on the way to producing weapons-grade uranium, which must be enriched to 90 percent. A February IAEA report confirmed that Iran had enriched uranium to 20 percent and that the IAEA could not rule out "current" work on weaponization.

What new sanctions are being considered?

Before the Iranian elections last year, Washington had made several diplomatic overtures to Iran. But now the equation has changed.

The US has proposed UN sanctions that would target the Revolutionary Guard and concentrate on insurance, shipping, and banking in Iran.

While Russia has shown increasing willingness to support new sanctions, China remains opposed. Both nations hold veto power. Iran is the second-largest supplier of oil to China, which has large investments in Iran's energy sector.

Recently Brazil, Lebanon, and Turkey, which do not have veto power, also voiced opposition to sanctions, undermining the effort to isolate China in its opposition to the measure.

Is it likely that the UN Security Council will pass new sanctions?

No. China's resistance makes it unlikely that new sanctions will be passed. What is more likely is a watered-down resolution that might add more Revolutionary Guard members to a blacklist and make recommendations against investment and business with Tehran.

He does not believe that economic sanctions would automatically cause a "rally around the flag" effect in Iran, swelling support for the government against a perceived Western oppression, but he says the West doesn't want to be painted by Mr. Ahmadinejad as responsible for the failure of his own economic policy.

"Sanctions must be done with a light touch, you've got to target them very specifically, and they have to reinforce the stupidity of the government," he says. But targeting the Revolutionary Guard has become more difficult since the Guard has become more involved in the general Iranian economy, says Ms. Elliott.

What's the next step for the US if UN sanctions don't pass?

The US and Europe will likely impose their own sanctions on Iran as they have done in the past; the US is already moving to do so. Both houses of Congress have passed legislation that would target companies that sell gas to Iran. But gasoline sanctions, says Elliott, "have a lot of risk," because they could allow Ahmadinejad's regime to end popular gasoline subsidies and blame it on the West.

Previous US sanctions on Iran have had little effect because they were unilateral, says Elliott, and sanctions that target companies who do business in Iran can also end up hurting US allies. Dr. Ansari says the US and the European Union need to enact robust policies to isolate the leaders responsible for human rights abuses as well as those associated with the nuclear program instead of relying on economic sanctions alone.

"I am very, very cautious on sanctions because I don't see them as a panacea," says Ansari. "The minute you see them as a panacea, then the minute they fail you're in a whole different ball game."